1. Field of the Invention
The present invention relates to a magnetic recording medium in which a magnetic metal thin film is formed on a nonmagnetic support as a magnetic layer.
2. Description of Related Art
Conventionally, as magnetic recording media, so-called coating type magnetic recording media have been widely used wherein the magnetic layer in the recording media is formed by applying to a nonmagnetic support a magnetic coating composition which consists of ferromagnetic powder, such as oxide magnetic powder or alloy magnetic powder, a binder, and an organic solvent.
On the other hand, there are increasing demands of recording at a higher density for a prolonged time, and, for meeting the demands, so-called ferromagnetic metal thin film type magnetic recording media are being used wherein the magnetic layer is formed by depositing a magnetic material containing a ferromagnetic metal directly onto a nonmagnetic support, such as a polyester film or a polyamide film, by, for example, a vacuum thin film forming technique, such as a vacuum deposition process, a sputtering process, or an ion plating process, or electroplating.
As a magnetic recording medium for meeting the demands of higher density recording, Japanese Patent Application Laid-Open Specification No. 11-203652 discloses a ferromagnetic metal thin film type magnetic recording medium having a magnetic layer which is optimized for reproduction with a helical scanning system using a magnetoresistance effect type replaying head (hereinafter, frequently referred to simply as “MR head”) having high sensitivity.
Generally, in a magnetic recording and replaying apparatus with a helical scanning system, a magnetic recording medium is always in contact with a magnetic head while traveling at a high speed. Therefore, the magnetic recording medium is very likely to suffer wearing or damage due to contact with the magnetic head. For solving this problem, in the ferromagnetic metal thin film type magnetic recording medium, a protecting film layer containing carbon or a lubricant layer is formed on the magnetic layer to improve the motion properties and robustness.
However, the magnetic head wears of f the protecting film or lubricant that is formed for realizing superior tape-transport properties and robustness during its movement, so that abrasion of the protecting film or lubricant, i.e. so-called generation of powder debris may occur. If such powder debris is accumulated, spacing between the magnetic recording medium and the magnetic head may be widened, thereby leading to a problem in that a predetermined signal output cannot be obtained. In addition, a problem is also encountered in that portions of the magnetic recording medium in which the protecting film or lubricant is worn of f become in contact with the magnetic head and wear off the magnetic head.
In recent years, for lowering the spacing loss and improving electromagnetic conversion characteristics, the ferromagnetic metal thin film type magnetic recording medium is improved in surface smoothness by a surface smoothing treatment, such as a supercalendering treatment. However, when the surface smoothness of the magnetic layer is extremely superior, the substantial contact area of the magnetic layer with the magnetic head becomes large, so that the coefficient of friction of the magnetic layer relative to the magnetic head tends to become much larger. Further, in accordance with an increase in the recording time, a period of time for rubbing the recording medium against the magnetic head is prolonged, and the scanning speed tends to be increased, thus promoting the generation of powder debris. Especially during the replaying operation in a still mode in which the same portion of a medium is continuously scanned repeatedly, the lubricant layer is difficult to recover by itself, and a load on each of the medium and the head is large, so that a dropout problem in that the output is suddenly lowered during the replay is likely to occur.
For example, Japanese Patent Application Laid-Open Specification No. 11-203652 discloses a magnetic recording medium which includes a magnetic layer optimized for the properties of the MR head used, and which uses, e.g., perfluoropolyether as a lubricant to exhibit superior lubricating effect under any conditions for use. However, when using perfluoropolyether as a lubricant, the generation of powder debris cannot be satisfactorily prevented, thus causing dropout of the output during replay or causing the head to wear down. Further, generally used perfluoropolyether having, for example, a hydroxyl group or a carboxyl group as a terminal group is difficult to be dissolved in an alcohol solvent or a hydrocarbon solvent, and therefore a fluorine solvent is inevitably used for dissolving the lubricant, leading to problems of in view of the preservation of the environment and productivity.
For resolving the problems, in the field of magnetic recording, typically, for preventing the protecting film from wearing off and removing the powder debris of the protecting film or lubricant, which is generated and deposited on the magnetic head, a method in which the hardness of the protecting film layer is increased so as to become physically abrasive to the magnetic head is used. However, when the hardness of the protecting film layer is increased, the protecting film layer may become too abrasive and wear off the magnetic head than it is necessary, thereby lowering the properties of the magnetic head.